This invention relates to a method and apparatus for compensating or correcting a positional error of numerical control apparatus which controls the movement of a movable member of a machine tool or the like.
In a numerically controlled machine tool a positional error occurs due to a pitch error or the like at the time of positioning the machine or passage control. For this reason, a numerical control apparatus is generally equipped with a position error compensating device. The position error is compensated for with the following method. For example, the range of movement of each shaft of the movable member of the machine is divided into a plurality of sections. For example, the range of movement in a X axis direction is divided into Xi (i=1, 2, 3, . . . ) sections and the amount of compensation EXi (i=1, 2, 3, . . . ) of each section is measured and stored in a memory device. When the present position, in the X axis direction of the movable member of the machine enters into a predetermined region AX1, the present position, in the X axis direction of the movable member of the machine is compensated for based on the amount of compensation EXi and in the directions in the positive direction or negative direction of movement of the machine.
In the prior art numerical value control apparatus, since a region in which the machine is positioned and a corresponding position compensation data are stored as a pair so as to compensate for the position error based on the stored data. As a consequence, the position error would vary in such a short distance, for example a movement error caused by a lead error which occurs at each rotation at the time of manufacturing a ball screw. Where such variation pattern occurs repeatedly in the range of movement of the movable member, it is necessary to increase the number of divisions (that is to decrease the length of the divided section) in order to effect a highly accurate compensation. Where there is a limit on the memory device that stores the compensation data, the number of compensation points would become deficient. Otherwise, it is necessary to increase the capacity of the memory device. Even when the memory device has an ample capacity it is necessary to store several times similar data occurring at each rotation of a ball screw over the entire range of movement.